<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width,initial-scale=1.0,user-scalable=no">
  <title>vue</title>
</head>

<body>
  <div id="app"></div>
</body>
<script>
  let htmlwidth = document.documentElement.clientWidth || document.body.clientWidth;
  let htmlDom = document.getElementsByTagName('html')[0];
  htmlDom.style.fontSize = htmlwidth / 20 + "px";
</script>

</html>